1. The Field of the Invention
The invention relates to a process for removing n-type doping impurities from liquid or gaseous substances or substance mixtures produced in the gas-phase deposition of silicon.
2. Description of the Prior Art
It is well known that for the production of polycrystalline high-purity silicon by gas-phase deposition, one must utilize substances having the highest possible purity. This is true whether the production process is for example by decomposition of volatile silicon compounds on heated silicon substrates by the Siemens process or on silicon particles in fluidized bed processes. In particular, the basic requirement is that electrically active impurities must be excluded as far as possible because these impurities may act as an undesired doping. In this connection, special attention must be paid to n-type doping impurities which may occur, for example, in the form of volatile compounds of elements of the fifth main group of the periodic system. These are, for example, halogens or, in particular, hydrogen compounds of phosphorus or arsenic.
Sources of such impurities may be, for example, starting materials such as the fragmented silicon raw material which is reacted with hydrogen chloride to produce trichlorosilane. The fragmented silicon is in general comminuted with steel jaw crushers and may be contaminated in the process with grit composed of, for example, phosphorus-containing steel. Furthermore, the silicon raw material itself may initially contain small traces of phosphorus or arsenic. Traces of such impurities may also be absorbed in small amounts from the stainless steel vessel walls or pipelines. This is because the starting or final products encountered in liquid or gas form in the silicon gas-phase deposition process are frequently very aggressive substances, such as silanes, halosilanes, silicon halides, hydrogen chloride or hydrogen.
The actual deposition reaction includes, for example, a mixture of hydrogen and liquid or gaseous trichlorosilane which mixture is reacted to form elementary silicon (in addition to large quantities of silicon tetrachloride, hydrogen chloride and also unreacted starting compounds). A lower amount of the n-type doping impurities are incorporated in the silicon deposited in this reaction owing to the various distribution equilibria and therefore undergo enrichment in the gas phase. This intrinsically favorable effect has, however, an undesirable result if the per se valuable gases produced as final products are intended to be recycled into the deposition process. This is because the enrichment then increases to an ever greater extent and finally results in an intolerable contamination.
An example of this is the so-called "tetra-conversion" in which the silicon tetrachloride produced is reconverted in a cyclic process into trichlorosilane which can then again be fed into the deposition reactor (see U.S. Pat. No. 4,536,642). Such an enrichment can also be produced in processes in which the unreacted hydrogen (for example, in accordance with U.S. Pat. No. 4,454,104) is fed back into the deposition reactor. In many cases, therefore, the use of such cyclic processes, which are desirable for cost reasons due to the considerable reduction in material consumption and also for environmental protection reasons, are either dispensed with entirely or used only occasionally in the gas-phase deposition of high-purity silicon.
A known process is disclosed in No. GB-A-975,000 by which liquid halosilanes or halogermanes can be freed of phosphorus-containing impurities such as phosphorus hydride or phosphorus trichloride. This patent teaches the conversion of the phosphorous containing impurities in a first step to the pentavalent state and then reacting them with added liquid titanium tetrachloride or tin tetrachloride, in which process a solid complex is formed which can eventually be removed. This process is restricted, however, to only liquid components and is expensive since it requires not only an oxidation stepbut also the careful separation of the liquid mixture formed after separating the solid complex. This process also makes no provision for the reuse of the reagents involved.